Polk RTA 12c crossover upgrade

eekarvon

I recently bought a beautiful real wood pair of RTA 12c’s. I decided these will be keepers. I really enjoy my mint 10 b’s but think these are a step up and would like to modify them for the best possible performance. I’ve already bought the Polk RD0194 tweeters. Next I would like to upgrade the crossovers. I don’t have the time or expertise to do them myself but am lucky enough to know a fellow that can. I was looking at the HF capacitors and noticed the yellow ones aren’t labeled. I would appreciate if any Polk guys out there had a parts list for the caps, resistors, and any other parts I may need. Would Sonicaps be significantly better than Dayton’s? I’m not knowledgeable on this but it seems like a complicated job? Any feedback I could get would be greatly appreciated!

Gardenstater

Yeah the PCB fabricators at Polk didn't care about exposing the numbers on those caps! I'm sure someone with an RTA 12c will help you out. If you remove one of the MW midbass drivers you might find a date and there may be a date on the crossover. The latest schematic we seem to have is for the RTA 12B and has a date of 11/26/84. The drivers were MW6501. I wouldn't doubt that something may be different in yours.

https://forum.polkaudio.com/discussion/38755/polk-audio-speaker-wiring-schematics-amp-more-all-models-except-sda#latest.

Most here will say that Sonicaps are better than Daytons. I've never heard anyone say the reverse. How much better I couldn't say. It's not hard to do really but you need to be familiar with desoldering and soldering techniques and use a good quality silver solder like Wonder Signature, Cardas Quad Eutectic, or WBT. Some single sided foam tape underneath along with a cable tie around the caps is a good idea. Space the resistors a little bit above the board (1/8" or so) to give them some breathing room and choose the 12W ones for less thermal noise.

Gardenstater

The Dayton Audio caps that Parts Express sells are probably rebranded Bennic XPP because they both have black tape and yellow body:

https://www.parts-express.com/dayton-audio-dmpc-33-33uf-250v-polypropylene-capacitor--027-441

http://www.bennic.com.tw/en/acocap/20xpp.asp

On this site the Bennic XPP was given a rating of 6.5 vs 8.5 for the Sonicap Gen I

http://www.humblehomemadehifi.com/Cap.html

pitdogg2

Dayton are Bennic that is correct.

Clarity cap is another decent cap on the low cost ratio

Gardenstater

pitdogg2 wrote: »

Dayton are Bennic that is correct.

Clarity cap is another decent cap on the low cost ratio

Yeah I was pretty close to 100% sure but thanks for the confirmation. I think an interesting takeaway is that on the Bennic site they have two higher end capacitor series SPP and FPP. It would be interesting to know how the prices of those compare to the ones that Parts Express carries (the XPP). FPP was rated on that capacitor site at 7 which is a higher rating than XPP (6.5) but not Sonicap Gen I (8.5).

Inquiring minds want to know

Gardenstater

One thing that bothers me is that Dayton does not claim their capacitor leads are pure copper where on the other hand the Bennic site absolutely says so. It seems foolish to not mention what is an obvious selling point.

eekarvon

Thanks so much for the replies. These 12C’s sound amazing in original condition and I’m amped to hear these gems modded out! My 10b’s are great also. It’s awesome to be able to buy great gear at great prices. I’ve had JBL 4311b’s and 4312a’s with the titanium tweeters and IMO the Polks still edged them at a fraction of the price.

drumminman

I have RTA 12C’s Which I’ve modded. When I get to the desktop computer I’ll see what I have that might be useful.
They are stellar speakers, and modding them takes them up several notches.

drumminman

I bought RTA 12C's new in the summer of 1984, and modded them in 2011. Mine came with the SL1000 tweeeters and the MW6600X mw's, and the circuit boards are stamped RTA 12B. There were lots of variations from the factory among the different generations of these speakers.

The mods I did were replace caps and resistors on both upper and lower Xovers; replaced the wiring with OCC solid core wire; Installed 4" wide strips of Black Hole 5 behind the MW's (kept the polyfill too); replaced the tweeters with RD-0194's; installed Hurricane nuts to fasten the MW's more firmly to the front baffle; replaced MW gaskets with Armacell adhesive foam; Dynamatted MW's and Passive Radiator; replaced the binding posts with Cardas copper ones; replaced the connector joining the lower Xover with the upper with a Neutrik.

For the Xovers I used all Sonicap with Mills resistors. I used the Schematic from 11/26/84 showing a fuse (which mine have) instead of the polyswitch as a guide. It has different MW's marked on it from what I have, but the values for the components on the board are the same.

The stuff I ordered from Sonicraft:

2 pairs - Cardas CCGG short binding posts
2 ea - Mills MRA12 7.5 ohm resistors
4 - 4.4uF
8 - 27uF 200VDC
4 - 27uF 200VDC (paired to equal 55uF, matched)

I bought this stuff during a sale so saved a bit, but the cost was still a shade less than $500. Geoff727 modded a pair years ago and posted pics of the work in progress. I searched for it but couldn't find it; maybe you can.

Gardenstater

https://forum.polkaudio.com/discussion/116480/rta-12cs-upgraded-for-forum-member-w-pics/p1

eekarvon

Thanks drumminman! Mine have the dreaded sl 2000 tweeters but honestly with my 33 band equalizer I can manage to get the sound closer to what I like.
Interesting how a pair of 12c’s would have a 12 b board. Apparently they just went with whatever they had in inventory. So if mine has a 12 c board could I still use your parts list? A pricey modification but In comparison you could easily spend more on an unmodified pair of great vintage speakers for sure. Considering I have a real wood excellent condition RTA 12’s the expense to me is justified. That oak veneer is pretty I must add. Thanks to everyone helping a newbie to the forum!

pitdogg2

Best to find out the values of your caps and resistors and duplicate. Pretty easy to heat up the solder point and pull up one side to read the values.
The yellow mylar caps are labeled differently but they are labeled with values and voltage.

drumminman

eekarvon wrote: »

Thanks drumminman! Mine have the dreaded sl 2000 tweeters but honestly with my 33 band equalizer I can manage to get the sound closer to what I like.
Interesting how a pair of 12c’s would have a 12 b board. Apparently they just went with whatever they had in inventory. So if mine has a 12 c board could I still use your parts list? A pricey modification but In comparison you could easily spend more on an unmodified pair of great vintage speakers for sure. Considering I have a real wood excellent condition RTA 12’s the expense to me is justified. That oak veneer is pretty I must add. Thanks to everyone helping a newbie to the forum!

Most likely the schematics linked will work, and the 12b boards are common in 12c's. As stated it's best to see what components you actually have on your boards.

The link Gardenstater provided will be enormously useful as Geoff727 has pics documenting each step. I also advise to mod one Xover at a time and use the other as a reference.

jwoods2112

jwoods2112

Well here one of mine is with the Audyn q4, cheapest boutique cap I could find, but did I need 400 V variety (weren’t many other options in this brand). Hmmmm, well I got them on there somehow. Let you know after “break in” period

jwoods2112

Unmodded speaker plays louder in treble noticeably brighter, hopefully means the new caps have a more refined sound but guessing I made them less efficient with the higher voltage caps?

F1nut

More refined sound.